Metal borides are currently of interest for use in bipolar plates of fuel cells. Metal borides may also find use in electronic, aerospace, and other energy areas.
For example, calcium hexaboride, a cubic metal boride belonging to the group 2A is a material attracting much attention due to its high hardness, high melting point, low density, chemical stability, and high electrical conductivity [References 1-3]. Calcium hexaboride can be exposed to high temperatures, while being able to provide surface protection in corrosive environments. The light weight, high density and highly conductive properties of calcium hexaboride make it a potential candidate for use in PEM Fuel Cells as a filler material for bipolar plates. Since bipolar plates account for 80% of the weight of a fuel cell stack, and 45% of stack cost [References 9, 10]; high quality CaB6 produced at low cost can potentially fulfill these responsibilities.
Various studies have investigated the synthesis of CaB6 powders using various methods as reported in [4-6], where researchers have employed the carbothermal method using boron carbide (B4C) as boron source to produce CaB6. However, the carbothermal method so far produced micron sized particles. The carbothermal reaction is limited by the contact area of reactants. Because of that the final product contains unacceptable quantities of unreacted metal oxides. Reaction time is very long. Reaction temperature is high. The carbothermal method produces large particles with wide size range. The product from this method requires subsequent chemical treatment and milling. Other methods may also used to produce CaB6 powders such as boron and calcium containing chlorine may be used to produce CaB6; however corrosive by products and stoichiometry are problem. [References 7-8].
The CaB6 precursors described herein are formed via pyrolysis of propylene. The low-cost method described herein, developed by Koc and Glatzmier, as briefly mentioned in [Reference 11], provides a pure form of carbon, which is amorphous, and provides excellent overall contact with the reacting powder.
There are no currently known methods of producing submicron-sized metal boride powders that are sufficiently pure enough for use. Such useful “pure” powders are compositionally pure (e.g., chemically pure and single phase), and are also within a narrow size distribution, spherical in shape, and free of agglomeration. Additionally, the known methods, which produce inappropriate powders, are not necessarily low-cost methods.